Question 64

Evaluate Statement I: 'At least three non-overlapping channels are available for transmissions.'
In the 2.4 GHz band defined by IEEE 802.11b/g/n, channels 1, 6, and 11 are non-overlapping channels (22 MHz width spaced 25 MHz apart). In the 5 GHz band (802.11a/n/ac), there are up to 24 non-overlapping channels. Thus, at least three non-overlapping channels are available. Statement I is TRUE.

Evaluate Statement II: 'The RTS-CTS mechanism is used for collision detection.'
In wireless networks, transmitting at full power masks any incoming collision signals, so collision detection (CD) cannot be implemented. IEEE 802.11 uses CSMA/CA (Collision Avoidance). The RTS-CTS handshake is used for collision avoidance and to solve the hidden terminal problem, not for collision detection. Statement II is FALSE.

Evaluate Statement III: 'Unicast frames are ACKed.'
Because of high bit error rates and the absence of collision detection, the 802.11 MAC layer uses stop-and-wait positive acknowledgements for all unicast data frames. Broadcast and multicast frames are not acknowledged, but unicast frames are explicitly ACKed. Statement III is TRUE.

Therefore, Statements I and III only are true.